Outdoors you have two choices:
Shoot the stock G31 in any of the following categories--Amateur Civilian, Competition, Master Stock. You will be shooting against fellow amateurs in Am/Civ and Competition.
Shoot with your aftermarket barrel in Masters/Unlimited, in which you will be competing against Masters and their full blown race guns.
You cannot use an aftermarket barrel in stock categories. Either way you go, you will need about 150 rounds to be safe. Maximum round count is 107, minimum is 81.